1. Field of the Invention
The present invention concerns a method for monitoring RF energy emission by a radio frequency apparatus as well as a corresponding radio frequency apparatus and a corresponding radio frequency monitoring apparatus for implementation of such a method. Moreover, the invention concerns a magnetic resonance tomography system with such a radio frequency apparatus.
2. Description of the Prior Art
Magnetic resonance imaging, based on examination of the nuclear magnetic resonance of protons of a body region, has become established as an imaging modality in the medical field. In this modality, a strong, stable, homogenous magnetic field is initially generated around the body region, causing a stable alignment of the protons in the appertaining body region. This stable alignment is then altered by radio frequency energy emitted in to the body region. This energetic simulation is ended and the magnetic resonance signals generated in the body are measured by suitable reception coils in order to make conclusions about the tissue in this body region.
A magnetic resonance tomography system includes a number of interacting components, each one of which requires the use of modern and complex technologies. A central element of a magnetic resonance tomography system is the radio frequency apparatus. This is responsible for the generation of the radio frequency pulses to be radiated into a body region. The radio frequency pulses output by a radio frequency power amplification device of the radio frequency apparatus of a magnetic resonance tomography system are supplied via a measurement device to a transmission coil that radiates the radio frequency pulses into a body region. “Transmission coil”, as used herein means an arbitrary antenna device with which the radio frequency pulses can be radiated.
With the development and establishment of magnetic resonance tomography systems, limit values that regulate the maximum radio frequency irradiation in the human body have been normalized to ensure the patient safety. A typical limit value for this is the maximal allowable SAR value (SAR=specific absorption rate).
To obtain these limit values, via the aforementioned measurement device measurement values are acquired that represent the power of the radio frequency pulses radiated by the transmission coil. Power-monitoring values are formed on the basis of a number of power measurement values. These power-monitoring values are then compared with fixed power limit value provided by a norm, this fixed power limit value being selected so that the predetermined SAR limit value is not exceeded. The radio frequency apparatus is automatically limited in terms of its function when a monitoring value exceeds the predetermined threshold.
Such a method is described, for example, in US 2002/0093336 A1. In order to increase safety, it is proposed to measure the radio frequency power and to form a number of sliding radio frequency power average values over respective different time intervals. Each of the time intervals is associated with its own switch threshold. When one of the sliding average values exceeds the associated switch threshold, the magnetic resonance measurement is aborted or modified.
A similar method is described in DE 101 53 320 A1. In order to prevent unwanted forced shutdowns or changes during the measurement due to overruns of the power limit values, predictions about the probable SAR values reached in a measurement are additionally made before the measurement. Expected limit value overruns are already known in advance due to these predictions, so that if necessary the measurement protocol can be changed in order to safely adhere to the limit values. In order to prevent with safety an overrun of the SAR limit value (set by statute) in the measurement, however, the actual radio frequency energy radiated by the system during a magnetic resonance measurement is also measured in order, if necessary, to shut down the radio frequency system given an overrun of the allowed, accumulated radio frequency energy within a predetermined time interval.
This means that the maximum allowable SAR was conventionally always converted into a maximum allowable power and this power limit value was monitored during a magnetic resonance measurement. The physiological effect of radio frequency energy on a human or animal body, however, depends on, among other things, the frequency and the coil type, i.e. on whether the coil emits in a circular or linearly-polarized manner or whether it is a volume or surface coil. Moreover, the effect also depends on the position of the coil on the body of the patient. In the conventional monitoring methods, examinations had to be done in part with immense safety margins with regard to the actual critical value in order to ensure 100% safety for the patient. This means that the allowable power limit value generally lies significantly lower than this is actually necessary to obtain the maximum exposure.
Since a lower image quality normally is associated with lower radio frequency power, it would be desirable to reduce the overly large safety margins. It must also be considered that a lower image quality ultimately leads to exposures that may not offer the desired diagnosis possibilities, or even to exposures having to be produced again, which in turn leads to a higher exposure of the patient. This problem occurs to a particular degree in what is known as multinuclear spectroscopy. Such multinuclear spectroscopy measurements are used to an increasing degree in magnetic resonance diagnostics. Instead of only one nucleus type being excited in the measurement; decouplings of other nuclei also occur in order to generate additional parameter images that can significantly improve the later evaluation of the measurement results. For this purpose, radio frequency energies must be radiated at various frequencies and in part also with various coils, i.e. on different transmission paths. Although, given the same irradiation power, the physiological effect and thus the radiated SAR is different over the various transmission paths, a correspondingly different parameterization has conventionally not been possible in the monitoring of the radio frequency pulses. This means that it has conventionally been assumed that all radio frequency pulses with the same energy or power cause the same exposure in the body. A conversion of the pulses or series of the pulse sequences planned by the measurement program into, for example, a SAR value and the consequential control of the radio frequency transmission device is not possible since the monitoring with regard to the SAR value must be autonomous in and may not rely on specifications from predictions of the measurement program that provides the pulse sequences for the measurement. The effect most harmful for the patient conventionally has been assumed for each measured radio frequency pulse in a “worst case” scenario. A simultaneous monitoring of the transmission power on various paths in one and the same measurement was previously not possible. A very early technical limit was therefore set for the applications and experiments—in particular with regard to multinuclear spectroscopy.